1. Field of the Invention
This invention relates generally to the field of position control, and more particularly to the use of stepper motors for fine position control.
2. Description of the Prior Art
Many tasks require fine position control, such as the positioning of a sensor such as a read/write head assembly in a magnetic tape or disk drive. Stepper motors are electromechanical devices developed for use in accurate and repeatable positioning systems which can be moved or stepped along discrete step or target positions in response to electrical pulses. Stepper motors include periodic stator magnet structures energizable by electric coils arranged so that the proper sequencing of coil energizations causes the periodic rotor magnet structure to be moved in a stepwise fashion from one target position to the next. Although there generally are position errors in positioning the rotor at each step, these errors are non- cumulative.
A typical stepper motor may have 24 steps per rotation, providing target positions at every 15.degree. of rotation. Stepper motors are commercially available which can provide full step rotations as small as 0.9.degree.. One approach to providing finer position control than the full step of a stepper motor has been the development of the technique known as microstepping in which the rotor is positioned in discrete sub-steps within each full step. In full step operation, only one coil would typically be energized at a time. In microstepping operation, two coils may be energized so that a new target position is created between the target positions of either coil energized individually. If the coils are energized equally--that is, with equal currents--the microstep position is midway between the full step positions.
By using predetermined unbalanced currents to energize the two coils, multiple microsteps within each full step can be achieved. In a microstepping operation providing eight microsteps per full step, for example, a 24 step stepper motor has 8.times.24 or 192 steps. This increases the stepper motor resolution from 24 full steps of 15.degree. each to 192 microsteps of only 1.875.degree. each. The relationship between current and the microstep target position is non-linear, so the exact coil energization currents required for microstepping are typically stored in and read from a look-up table.
Conventional stepper motor systems, whether in full step or microstepping operation, provide only a finite number of discrete, spatially separated target positions in response to pulse actuation and are therefore considered digital positioning systems. Furthermore, at each target, there is generally some positional error that cannot be controlled. The torque delivered in a conventional stepper motor is proportional to the product of the current in the coil and the sine of the product of the angular distance of the rotor from the target position with the full number of steps available from the stepper motor. The torque is therefore non-linearly proportional to displacement, with a torque null at the target position. The torque null at the target position results in poor performance in the exact position in which high performance is most desired.
This non-linear relationship between torque and position makes a conventionally driven stepper motor a poor candidate for improving position control by analog servo control because classical linear servo analysis is valid only for linear systems. In addition, the torque null at the target position also makes a conventionally driven stepper motor a poor candidate for improvement by analog servo control because the target positions are at torque nulls. The resultant small signal gain approaches zero in the vicinity of the target, making conditionally stable servo systems unattractive because of the high potential for oscillation about the target.
What is needed is a technique for permitting finer position control from stepper motors than is available with conventional full step and microstepping operations.